Superalloy coating composition

ABSTRACT

Coatings for iron-, nickel- and cobalt-base superalloys. The coatings are applied in order to provide good oxidation/sulfidation and thermal fatigue resistance for the substrates to which the coatings are applied. The coatings consist essentially of, by weight, 10 to 50% chromium, 3 to 15% aluminum, 1.0 to 15% metal mixture from the group consisting of tantalum, tungsten, manganese and combinations thereof, up to 5% reactive metal from the group consisting of lanthanum, yttrium and other rare earth elements, up to 5 percent of rare earth and/or refractory metal oxide particles, and the balance selected from the group consisting of nickel, cobalt and iron, and combinations thereof. Additions of titanium up to 5% and noble metals up to 15% are also contemplated. Tantalum makes up at least 20% of the metal mixture or 0.5% of the total coating weight, whichever is greater. Tungsten, manganese, or a combination thereof, make up at least 0.5% of the total coating.

RELATED APPLICATION

This application is a continuation-in-part of applicants' copendingapplication Ser. No. 43,146, filed May 29, 1979, entitled "HighTemperature Oxidation and Sulfidation Resistant Coating", and nowabandoned.

BACKGROUND OF THE INVENTION

This invention is concerned with coatings adapted to significantlyimprove the elevated temperature corrosion resistance of articlescomposed of iron-, cobalt- or nickel-based superalloys whereby moresatisfactory performance and longer life for such articles can beobtained.

Elevated temperature exposure of metal articles is experienced in manysituations. Metal components are subjected to such conditions, forexample, in various aerospace applications and in land and marineoperations such as in the case of components utilized in gas turbineengines.

In such applications, it is important to provide some means forpreventing undue oxidation/sulfidation of the components involved sincesuch corrosion can materially shorten the useful life of the components.Deterioration of components can also create significant performance andsafety problems.

Various alloys, including most superalloys, are characterized by adegree of corrosion resistance, however, such resistance issignificantly decreased when unprotected superalloy components areexposed at the operating temperatures involved in certain systems. Forthat reason, such components have been provided with coatings, such asaluminide coatings, which increase the corrosion resistance at elevatedoperating temperatures.

Aluminide coatings are applied by methods such as the pack cementationprocess. In this process, the substrate chemistry and the processingtemperature exert a major influence on coating chemistry, thickness andproperties. Specifically, the coatings comprise a hard, brittle outerlayer and a hard, brittle multi-phase sublayer that can crack whensubjected to mechanically or thermally induced strain. This leads topoor fatigue properties, and the cracks can also materially reduce thecorrosion resistance of the coated components.

Another class of coatings is the MCrAlY overlay coatings where M standsfor a transition metal element such as iron, cobalt or nickel. MCrAlYcoatings have been shown to have an advantage over aluminide coatings inproviding extended life to turbine components. Specifically, MCrAlYcoatings generally demonstrate greater corrosion resistance thanaluminide coatings and also greatly superior ductility.

Presently, these MCrAlY coatings are applied by vacuum physical vapordeposition. However, the fundamental nature of the vacuum physical vapordeposition process limits the composition of the coating that can beapplied to an article. Specifically, with a single multi-element source,it is very difficult to deposit MCrAlY type coatings which contain otherelements that have either very low or very high vapor pressures.Resorting to dual or multiple sources introduces a further degree ofcomplexity to an already complex process which is undesirable from aproduction standpoint.

Another technique of applying MCrAlY coatings is plasma spraying. Inplasma spraying, the heated alloy particles corresponding to the desiredcoating composition are impinged on the preheated surface of the metalarticle at very high velocity and temperature. Such particles, uponcontact with the metal article surface or with other applied particles,deform plastically and fuse and bond to the surface or to the otherapplied particles, thus producing a dense and adherent coating. Plasmaspraying is particularly desirable since it is a generally less costlytechnique for producing the overlay coating and is not restricted byvapor pressure limitations as is the case with the vacuum physical vapordeposition processes.

Other attempts at improving elevated temperature corrosion resistanceare described in U.S. Pat. No. 4,145,481, issued on Mar. 20, 1979. Thisprocess involves the location of a MCrAlY coating over a substrate toprovide an overlay, and an aluminide coating was then added as an outerlayer. This technique attempts to achieve the advantages of theductility of the MCrAlY and the resistance to elevated temperaturecorrosion of the aluminide. Copending application Ser. No. 847,253,filed on Oct. 31, 1977, takes the approach of utilizing first and secondMCrAlY-type coatings on a substrate. A first coating is intended toprovide a ductile layer with the second coating providing a layer havinga greater resistance to elevated temperature corrosion.

Still other approaches, particularly from the standpoint of alloyingingredients and application techniques are described in the followingU.S. patents:

    ______________________________________                                        Inventor      U.S. Pat. No.                                                                              Date Of Issue                                      ______________________________________                                        Gedwill, et al.                                                                             3,849,865    Nov. 26, 1974                                      Gedwill, et al.                                                                             3,869,779    Mar. 11, 1975                                      Hecht, et al. 3,928,026    Dec. 23, 1975                                      Bessen        3,957,454    May 18, 1976                                       Preston       4,005,989    Feb. 1, 1977                                       ______________________________________                                    

In view of the fact that increasingly greater demands are placed onperformance, particularly for components subject to extreme temperatureconditions, it is desirable to provide even greater improvements in thecapabilities of coatings of the type described. The demand for requisiteductility while maintaining resistance to the corrosive effects oftemperature and atmosphere is particularly critical.

Oxidation-sulfidation resistance and thermal fatigue resistance attemperatures above 1400° F. is of great importance. Coatings suitablefor metal components which are subjected to a relatively low temperature(less than 1400° F.) corrosive environment are, however, also of greatvalue.

The low temperature corrosive environment, in particular, refers to theconditions that exist in liquid-fueled turbines burning fuel high insulfur and vanadium content and operating in a marine environment.Substantial sulfidation (hot corrosion) has been observed in these typesof engines, especially when they are operated at low power settings (lowtemperature). SO₃ has been identified as an agent that can beresponsible for this type of attack.

In such applications, it is important to provide some means ofpreventing the catastrophic corrosion since such corrosion canmaterially shorten the useful life of the components. Deterioration ofcomponents can also create significant performance and safety problems.

Some attempts have been made to develop coating compositions to combatthe problem of "low temperature" corrosion occurring below 1400° F. Thefollowing U.S. patents describe compositions and application techniqueswhich might be used for this application:

    ______________________________________                                        U.S. Pat. No.  Date of Issue Patentee                                         ______________________________________                                        4,022,587      May 10, 1977  Wlodek                                           4,088,479      May 9, 1978   Spengler                                         4,101,715      July 18, 1978 Rairden                                          ______________________________________                                    

These compositions are simply MCrAlY coatings. In view of the energyshortage, resulting in gas turbine engines burning "dirty" fuelcontaining large amounts of sulfur and vanadium, it is desirable toprovide even greater improvements in the capabilities of coatings toprovide corrosion resistance at temperatures below approximately 1400°F.

SUMMARY OF THE INVENTION

The coating compositions of this invention are particularly resistant tooxidation-sulfidation at elevated temperatures, are otherwise highlyefficient in their performance at these temperatures, and are wellsuited for application to substrates by plasma spraying. In the broadestsense, the coatings consist essentially of from 10 to 50% chromium, 3 to15% aluminum, a metal mixture from the group consisting of tantalum,tungsten and manganese and combinations thereof, and the balanceselected from the group consisting of nickel, cobalt and iron andcombinations thereof. The mixture of tantalum, tungsten and manganese ispresent in amounts from 1 to 15%, and tantalum comprises at leastone-fifth by weight of the mixture, or at least 0.5% of the totalcoating weight, whichever is greater. The balance of the mixtureconsists of at least 0.5% manganese or tungsten, or combinationsthereof.

Optionally, the coating may have up to 5% reactive metal from the groupconsisting of lanthanum, yttrium and the other rare earths. Also theaddition of rare earth and/or refractory metal oxide particles to theaforementioned coating composition is contemplated; these ingredientspreferably being individually utilized in amounts from about 0.05 up toabout 2.0% by weight. Such additions can be beneficial to the over-allprotective response of the coating because the metal oxide particlesassist in pinning protective oxide scales. This pinning phenomenonresults in superior adherence (less spalling) of the protective scale,thus increasing the overall coating life. Additions of titanium up toabout 5% and of noble metals up to about 15% are also contemplated.

It will be appreciated that in using the term "coating", reference toapplication of material to a substrate surface is generally intended.Use of the material as an abradable sealing material is, for example,contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a 500× magnification of the microstructure of a coatingand substrate particularly suited for performance at high temperatures;

FIG. 2 comprises a 500× magnification of the microstructure of a coatingand substrate particularly suited for performance at low temperatures;

FIG. 3 is a plan view of a test specimen having temperature profilessuperimposed thereon; and,

FIG. 4 is an end view of the test specimen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As set forth in the foregoing summary, the invention relates to acoating composition for superalloy substrates. Protection againstoxidation at very high temperatures (in the order of 1400° F. andhigher) is particularly important, and a coating particularly suited tothis consists essentially of from 5 to 35% cobalt, 10 to 35% chromium, 5to 15% aluminum, a metal mixture from the group consisting of tantalum,tungsten and manganese and combinations thereof, and the balance nickel.The mixture is present in amounts from 1 to 15%, and tantalum representsat least one-fifth by weight of the mixture, or at least 0.5 of thetotal coating weight, whichever is greater. The balance of the mixtureconsists of at least 0.5% manganese or tungsten, or combinationsthereof.

This coating may include alloying elements for various purposes, forexample up to 5% reactive metal consisting of lanthanum, yttrium and theother rare earths. In the preferred form of the invention, theseelements are utilized in an amount between 1.0 and 3.0% by weight of thecoating composition.

Rare earth of refractory metal oxide particles in amounts beneficial tothe over-all protective aspects of the coating are also contemplatedbecause they assist relative to the pinning phenomenon. As indicated,these additions are preferably in the range of 0.5 to 2.0% by weight.

A similar inprovement in the coating life can be achieved by theaddition of up to about 15 weight percent of a noble metal selected fromthe group consisting of platinum, rhodium or palladium. An addition ofthis type also improves the diffusional stability of the coating.

In formulating the metal mixture which makes up a critical portion ofthe coating composition, it is preferred that the mixture containbetween 2 and 5% tantalum along with between 2 and 5% of materialcomprising tungsten, manganese or a combination thereof. It has beenfound, however, that the amount of tungsten preferably does not exceed1.5% by weight. The other ingredients of the coating composition of thistype are preferably employed within the limits of 10 to 20% cobalt, 15to 25% chromium and 10 to 14% aluminum.

It is contemplated that the coating composition of this invention formthe corrosion resistant outer layer of the two layer MCrAlY coatingdescribed in the aforementioned patent application Ser. No. 847,253, nowpatent No. 4,198,442.

The composition may also form the inner layer or the bond coat for a twolayer or graded thermal barrier coating which is used to reduce metaloperating temperatures and the effects of thermal transients in gasturbine engines. In such applications, the bond coat must be similar tothe substrate, and the composition of the ceramic/metallic two layer orgraded coating system must be such that stresses caused by thermalexpansion mismatch between the bond coat and the compatible oxide outerlayer are minimized. In addition, the inner layer must be fairly denseand the surface of this undercoat must be rough enough to provide anadherent surface for the oxide overcoat. Plasma sprayed compositionssatisfy these requirements, and hence, are ideally suited for thisapplication.

As noted, the above narrower composition ranges are best suited forhigher temperatures although compositions within the broader ranges haveutility at such temperatures. With reference to compositions suited forthe coating of superalloy substrates where operating temperatures areless than about 1400° F., the broader ranges for percentages ofingredients are recommended. Specifically, higher chromium content isdesirable in certain instances with from 10 to 50% by weight chromiumbeing an appropriate range. On the other hand, lower percentages ofaluminum can be utilized and, therefore, an aluminum range from 3 to 15%is appropriate. Finally, titanium in amounts up to about 5% by weight isdesirably included.

The intermediate temperature coating compositions may comprise eithernickel, cobalt or iron base compositions with combinations of theseingredients being suitable but not required. Otherwise, the coatingcompositions include additions of a metal mixture comprising tantalumcombined with tungsten or manganese or a combination thereof withpreferred inclusions of a reactive metal and, alternatively, inclusionsof rare earth and/or refractory metal oxide particles. In each instance,the amounts of these ingredients are utilized in the same ranges asexpressed relative to the high temperature coatings.

Other uses for the compositions of the invention will be apparent tothose skilled in the art, and it will also be appreciated that otheralloying elements may be employed in the coatings in accepted amountsand in accordance with known practices.

The utilization of plasma spray techniques to deposit the coatingcompositions is preferred. The wide differences in the evaporation rate(or vapor pressure) between high vapor pressure elements like manganeseor aluminum and low vapor pressure elements like tantalum or tungstenmakes the deposition and composition control of this coating by vacuumphysical vapor deposition difficult, if not impossible. In addition,compositions in accordance with this invention produce very densecoatings after plasma spraying. It will be appreciated, however, thatprocess improvements or modifications in methods such as physical vapordeposition or ion plating could make coating by these methods possible,and the use of these methods is therefore contemplated. Additionally,techniques like sputtering or slurry sintering may also be considered.

After deposition of the coating, the coated articles preferably aresubjected to an elevated temperature treatment in the range of 1900° F.to 2200° F. for a duration of one to 10 hours in an inert atmosphere(for example, in a vacuum or in an argon atmosphere) to promoteinterfacial bonding.

To illustrate the practice of the invention, a typical nickel-basesuperalloy of the type used in gas turbine engines, known as IN738, andhaving a nominal composition of 0.09% C, 16.0% Cr, 8.5% Co. 1.7% Mo,2.5% W, 1.7% Ta, 3.5% Ti, 3.5% Al, 0.01% B, 0.03% Zr and the balance Ni,was provided as one substrate. A typical cobalt-base superalloy of thetype used in gas turbine engines, known as Mar-M509 and having a nominalcomposition of 0.60% C, 23.4% Cr. 10.0% Ni, 7% W, 3.5% Ta, 0.23% Ti,0.01% B, 0.45% Zr, 1.5% Fe, 0.10% Mn, 0.40% Si and the balance Co,provided a second substrate for testing.

A first series of coatings was produced by plasma spraying prealloyedpowders. These powders were sprayed in a plasma arc (>Mach 3 velocity)using argon and helium as primary and secondary gases, respectively.Spraying was performed in a chamber maintained at a dynamic pressure of55 Torr. The process parameters were:

    ______________________________________                                        Gun to workpiece distance                                                                         16 in.                                                    Primary gas (argon) 370 CFH at 225 psi                                        Secondary gas (helium)                                                                            150 CFH at 250 psi                                        Voltage             50-52 volts                                               Current             1400-1440 amps                                            Powder flow         0.07 lb./min.                                             Carrier gas (argon) 25 CFH at 100 psi                                         Time for deposition 45 sec.                                                   ______________________________________                                    

The articles were then subjected to heat treatment in a vacuum for fourhours at 1975° F.

The following table illustrates the compositions, tested and the testresults:

                                      TABLE I                                     __________________________________________________________________________    PROPERTIES OF MCrAlY COATINGS                                                                                                     IMPACT ENERGY                                                                 REQUIRED                                             AVERAGE LIFE.sup.(1)                                                                         CRACKING  TO PRODUCT                                           (HOURS)        AFTER     CRACKS.sup.(2)            COATING                                                                              COMPOSITION (WI %)  IN738   MAR-M509                                                                             WATER SPRAY                                                                             OR CHIPS                  SYSTEM Ni Co                                                                              Cr                                                                              Al                                                                              Ta                                                                              Mn W La                                                                              Y SUBSTRATE                                                                             SUBSTRATE                                                                            QUENCH TEST                                                                             (IN.-LBS)                 __________________________________________________________________________    UTC.sup.(3)                                                                   NiCoCrAlY                                                                            Bal                                                                              23                                                                              18                                                                              13         0.3                                                                             100     190    No        3.0                       MDC-35A                                                                              Bal                                                                              15                                                                              20                                                                              12                                                                              2.5    0.5 107            No        3.0                       MDC-34H                                                                              Bal                                                                              10                                                                              20                                                                              17         0.6                                                                             186            Yes       1.0                       MDC-1A Simple Aluminide      23.sup.(4)   Yes       0.5                       LDC-2E Platinum Aluminide  135            Yes       2.0                       MDC-35B                                                                              Bal                                                                              15                                                                              20                                                                              12                                                                              2.5  1.5                                                                             0.5 124     237    No        3.0                       MDC-35C                                                                              Bal                                                                              15                                                                              20                                                                              12  2.5  0.5 110            No        3.0                       MDC-35D                                                                              Bal                                                                              15                                                                              20                                                                              12                                                                              2.5                                                                             2.5  0.5 175     238    No        2.0-3.0                   MDC-35E                                                                              Bal                                                                              15                                                                              20                                                                              12                                                                              2.5  1.5 0.5                                                                             125            No        3.0                       MDC-35M                                                                              Bal                                                                              21                                                                              16                                                                              12                                                                              2.5                                                                             1.7  1.0 230            No        3.5-4.0                   __________________________________________________________________________     .sup.(1) Rig Cycle: 2100° F./2 Min. + 1750° F./4 Min. +         2100° F./2 Min. + Cool/2 Min. (5 ppm salt).                            .sup.(2) Results obtained from drop weight test.                              .sup.(3) Composition conforming to United Technologies Pat. No. 3,928,026     .sup.(4) Result from one test.                                           

A 500× photomicrograph of one of the coatings (MDC-35D) is shown in FIG.1; the thicknesses of the coatings typically were 0.004 inches, however,it is contemplated that the coating thickness vary between 0.0001 and0.1 inches. The optical micrograph reveals the presence of a ductilematrix of gamma (Ni, Cr) containing a dispersion of beta (Ni, Co) Alintermetallic compound. The proportion of these two phases was about thesame in all the MCrAlY coatings listed in Table I with the exception ofMDC-34H; this high Al MCrAlY coating contained more of the beta phase.Electron microprobe analysis showed that the coating chemistry was notvery much different from that of the chemistry of the powder.

The performance of the articles coated pursuant to this example wasevaluated using a 0.7 Mach burner rig. The testing cycle was 2100° F./2minutes; 1750° F./4 minutes; 2100° F./2 minutes; air cool/2 minutes.Five (5) ppm salt solution was injected into the combustion products ofJP5 fuel containing 0.2% sulfur. This cycle closely simulates the gasturbine engine environment for turbine vanes and blades, it highlightsthe oxidation phenomenon, and it imposes significant thermal stresses onthe protection system.

It is seen from Table I that coatings produced in accordance with thisinvention exhibit substantially improved performance compared to asimple MCrAlY type system (UTC NiCoCrAlY, U.S. Pat. No. 3,928,026). Inaddition, the corrosion resistance of the relatively low aluminumcontent material is similar to that of a very high aluminum contentMCrAlY coating like MDC-34H which had more of the beta phase. A typicalhigh Al content MCrAlY will have good oxidation resistance but poorductility because of the high amount of beta phase; whereas, a low Alcontent MCrAlY will have good ductility but relatively poor oxidationresistance. Coatings produced in accordance with this inventiondemonstrate excellent oxidation resistance and, because of theirrelatively low aluminum content, exhibit excellent ductility as will bediscussed. The coatings also reveal improved oxidation resistancecompared to an advanced platinum aluminide coating like LDC-2E, and asimple aluminide coating such as MCD-1A.

The performance of the articles coated pursuant to this example was alsoevaluated by means of a water spray quench test and drop weight impacttesting. The former test is a measure of coating ductility and consistsof heating the coated airfoil sections to 2100° F.±100° F., holding thearticles at this temperature for time periods of 15 minutes to twohours, and then quenching them in a water spray. It is to be noted thatthe thermal strains that are generated in this type of test are lesssevere than those encountered in cooled aircraft engine gas turbineblades and may be similar to those experienced in other types of gasturbine blades. The latter test is also a measure of coating ductility,highlighting the handling characteristics of the coated parts. Itconsists of dropping a one pound indentor from several heights onto thetrailing edge of a coated airfoil section with the energy of impactbeing equal to the height in terms of inch-lbs. The tested specimens areevaluated, using a stereo microscope at 20× magnification, forappearance of defects (chips and cracks). The energy of impact necessaryto produce cracks or chips on the trailing edges is taken as a measureof coating ductility. The higher this energy, the greater the coatingductility.

The results from these two tests are also shown in Table I. It is seenthat articles coated pursuant to this example do not show any cracksafter the water spray quench test unlike the high Al MCrAlY or thealuminides thereby confirming the ductility of coatings in accordancewith this invention. Drop weight tests done on the trailing edges ofcoated airfoil sections indicate that articles coated pursuant to thisexample are able to withstand greater impact energy than high Al MCrAlYor aluminide coated articles. This also confirms that coatings inaccordance with this patent are ductile.

The Table also illustrates the value of the use of the metal mixture ofthe invention. Thus, MDC-35A contains only tantalum and MDC-35C containsonly manganese, while MDC-35B, D, E and M contain the mixture. Thelatter exhibit significantly improved coating life.

As indicated, the foregoing series of coatings illustrate theapplication of the invention for high temperature operations. A secondseries of coatings was developed for purposes of illustrating theapplication of the invention to intermediate temperature applications.The IN738 and MAR-M509 substrates previously referred to were also usedin association with these coatings. The coatings were plasma sprayedonto the substrates in accordance with the parameters previouslydescribed and the same heat treatment in a vacuum for four hours at1975° F. was utilized.

A 500× photomicrograph of one of the coatings (MDC-36D) is shown in FIG.2. The thickness of the coatings typically was between 0.004 to 0.006inches, and it is contemplated that the coating thickness vary between0.0001 and 0.1 inches. Electron microprobe analysis revealed that thecoating chemistry is substantially the same as that of the powder.

The performance of the articles coated pursuant to this example wasevaluated using a 0.7 Mach burner rig. The testing cycle was 1750° F./2minutes; 1450° F./4 minutes; 1750° F./2 minutes; air cool/2 minutes. Asshown in FIG. 3, the peak temperature of 1750° F. generated a spectrumof temperatures in the paddle test specimen, there being a variation ofabout 200° F. (1550° F. to 1750° F.) over the entire surface of thespecimen. The same variation and isotherm pattern developed at the lowerend of the cycle (1450° F.), and the coatings that were tested weretherefore exposed to temperatures from 1250° F. to 1750° F. This is oneof the conditions necessary for the aforementioned corrosion to takeplace, and the other conditions were created by adjusting the sulfurcontent in the fuel and by injecting salt solution into the combustionproducts. The fuel used was JP5 doped with ditertiary butyl disulfide toobtain 0.3% sulfur; and fifty (50) ppm salt was injected into thecombustion products. These conditions closely simulate the gas turbineengine environment for turbine blades and vanes in marine applications,and they highlight the sulfidation (hot corrosion) phenomenon whileimposing significant thermal stresses on the protection system.

The results of this test are shown in Table II:

                                      TABLE II                                    __________________________________________________________________________    BURNER RIG TEST RESULTS OF HIGH Cr LOW Al                                     MODIFIED MCrAlY COATINGS                                                                                      AVERAGE                                       COATING  COMPOSITION (WI %)     LIFE.sup.(2)                                  SYSTEM.sup.(1)                                                                         Ni Co Cr                                                                              Al                                                                              Ti                                                                              Ta                                                                              Mn W La                                                                              Y (HRS.)                                        __________________________________________________________________________    UTC CoCrAlY.sup.(3)                                                                       Bal                                                                              23                                                                              13           0.6                                                                              912                                          GE CoCrAlY.sup.(4)                                                                        Bal                                                                              29                                                                              6            1 1020                                          MDC-34U  29 Bal                                                                              26                                                                              6            0.6                                                                              936                                          MDC-34Y  Bal                                                                              10 20                                                                              6            0.6                                                                             1445                                          MDC-36D     Bal                                                                              30                                                                              9   2.5                                                                             1.7  1.0 1500↑.sup.(5)                           MDC-36E  Bal                                                                               5 30                                                                              8 2.0                                                                             2.5                                                                             1.7  1.0 1700↑.sup.(5)                           __________________________________________________________________________     .sup.(1) On both IN738 and MARM509 substrates                                 .sup.(2) Rig cycle: 1750° F./2 min + 1450° F./4 min +           1750° F./2 min + cool/2 min (50 ppm salt)                              .sup.(3) Composition conforming to United Technologies' Pat. No. 3,676,08     .sup.(4) Composition conforming to General Electric Co. Pat. No. 4,101,71     .sup.(5) ↑ denotes "not yet failed                                 

The term "average life" mentioned in the above refers to the approximatenumber of hours of test prior to the formation of an observablesubstrate metal oxide (which indicates that the coating has beenpenetrated under the burner rig test conditions) anywhere on the paddlespecimen. It is seen that coatings produced in accordance with thisinvention unexpectedly exhibit substantially improved performancecompared to a simple MCrAlY system such as the CoCrAlY of U.S. Pat. No.3,676,085, or the CoCrAlY specimen, MDC-31C. The test also revealed thatadditions of Ta, Mn and Ti improve the corrosion resistance in a fashionsimilar to the improvements observed in Table I.

Although the performance of these high Cr compositions was the same onboth IN738 and MAR-M509 alloys, the microstructures were different. HighCr coating compositions on cobalt-base substrates tended to form acontinuous layer of carbide (probably M₂₃ C₆) at the interface. Thetendency of this carbide formation was reduced by increasing the amountof Co in the coating. Coatings on Ni-base alloys were free of carbideformation.

The performance of the articles coated pursuant to this example was alsoevaluated by means of water spray quench testing and drop weight impacttesting of the types previously described. Coating ductility isimportant for gas turbine engine applications to ensure that themechanical properties of the substrate alloy are not compromised.

The results from the first test showed that articles coated pursuant tothis example do not show any cracks after the water spray quench testunlike the UTC CoCrAlY coating, thereby showing that these coatings areductile. Drop weight tests done on the trailing edges of coated airfoilsections also indicate that articles coated pursuant to this example areable to withstand greater impact energy than the UTC CoCrAlY coating.This also confirms that coatings in accordance with this concept areductile.

The coating compositions falling within the scope of this invention aresuitable for a wide variety of superalloy substrates, and the particularexamples of substrates referred to herein are not to be consideredlimiting. Thus, any substrate which can be satisfactorily coated withthe composition of this invention by means of plasma spraying or anyother suitable coating technique, and which will retain the coating in asatisfactory manner where elevated temperature performance iscontemplated, will be suitable.

Coating compositions falling within the broader ranges expressed hereinare generally useful for applications where sulfidation and/or oxidationresistance is desired. As explained, applicant's have also discoveredthat certain more limited ranges within the broad ranges provideparticularly suitable performance for elevated temperature applications.The following claims are presented with this general understanding inmind.

It will be understood that various changes and modifications notspecifically referred to herein may be made in the invention describedwithout departing from the spirit of the invention particularly asdefined in the following claims.

We claim:
 1. A coating composition for application to nickel, cobalt,and iron base superalloys consisting essentially by weight of from 10 to50% chromium, 3 to 15% aluminum, up to 1.5% tungsten, 1 to 15% of ametal mixture, and the balance selected from the group consisting ofnickel, cobalt and iron, and combinations thereof, said metal mixtureconsisting essentially of at least 20% by weight tantalum and thebalance manganese, said coating containing at least about 0.5% by weighttantalum and at least about 0.5% by weight manganese.
 2. A coatingcomposition for application to nickel, cobalt and iron base superalloysto be exposed to temperatures in excess of about 1400° F. consistingessentially by weight of from 5 to 35% cobalt, 10 to 35% chromium, 5 to15% aluminum, up to 1.5% tungsten, 1 to 15% of a metal mixture, and thebalance nickel, said metal mixture consisting essentially of at least20% by weight tantalum and the balance manganese, said coatingcontaining at least about 0.5% by weight tantalum and at least about0.5% by weight manganese.
 3. A composition in accordance with claim 1 or2 comprising up to 5% by weight of a reactive metal selected from thegroup consisting of lanthanum, yttrium and the other rare earths.
 4. Acomposition in accordance with claim 3 wherein said reactive metal ispresent in an amount between 1 and 3% by weight of the composition.
 5. Acomposition in accordance with claim 1 or 2 including up to 5% by weightof a member selected from the group consisting of rare earth oxideparticles and refractory metal oxide particles.
 6. A composition inaccordance with claim 5 wherein said oxide particles are present in anamount between 0.05 and 2.0% by weight.
 7. A composition in accordancewith claim 1 or 2 wherein the tantalum is present in an amount between2.0 and 5.0% by weight, and including from 2.0 to 5.0% by weight oftungsten and manganese.
 8. A composition in accordance with claim 1 or 2including from 10 to 20% by weight cobalt, from 15 to 25% by weightchromium, and from 10 to 14% by weight aluminum.
 9. A composition inaccordance with claim 1 or 2 including from about 0.05 up to about 15percent by weight of a noble metal selected from the group consisting ofplatinum, rhodium or palladium.
 10. A composition in accordance withclaim 1 including up to about 5% by weight titanium.